Ab initio study of ABiO3 (A=Ba, Sr, Ca) under high pressure

Using ab initio crystal structure prediction we study the high-pressure phase diagram of ABiO(3) bismuthates (A = Ba, Sr, Ca) in a pressure range up to 100 GPa. All compounds show a transition from the low-pressure perovskite structure to highly distorted, low-symmetry phases at high pressures (PD transition), and remain charge-disproportionated and insulating up to the highest pressure studied. The PD transition at high pressures in bismuthates can be understood as a combined effect of steric arguments and of the strong tendency of bismuth to charge-disproportionation. In fact, distorted structures permit to achieve a very efficient atomic packing, and at the same time, to have Bi-O bonds of different lengths. The shift of the PD transition to higher pressures with increasing cation size within the ABiO(3) series can be explained in terms of chemical pressure.